This invention relates to a method of driving optical switch elements using a transparent high dielectric material, which is used for a printing portion of a printer of a duplicating machine, a printing portion of a facsimile, or for a display.
An optical switch element using transparent ceramics (PLZT) is well known as an optical switch element using a high dielectric material.
On the other hand, an optical switch element using a chiral smectic liquid crystal as a high dielectric material has recently drawn increasing attention, and hence this switch element will be described. Chiral smectic C and H liquid crystal materials, for example, inherently have a ferroelectric property. In the present specification, the term "high dielectric material" has been used to refer to ferroelectric liquid crystal materials.
In order to clarify the chiral smectic liquid crystal, Table 1 tabulates chemical structures and phase transition points of chiral smectic C liquid crystal SmC (DOBAMBC, OOBAMBCC) and chiral smectic H liquid crystal SmH (HOBACPC).
TABLE 1 __________________________________________________________________________ chemical structure, name (common name) phase transition point __________________________________________________________________________ ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## __________________________________________________________________________
Next, FIG. 10 shows the electrolytic response of these chiral smectic liquid crystal molecules (which will be hereinafter called the "liquid crystal molecules" unless specified otherwise). As shown in FIG. 10, the liquid crystal molecules 2 have a so-called "twist structure" around a spiral axis 1 under the state in which an electric field is not applied (E=0). When an electric field E exceeding a critical electric field E.sub.c, which is determined by the properties of the liquid crystal (such as spontaneous polarization, twist viscosity), is applied to the liquid crystal molecules from an orthogonal direction with respect to the spiral axis 1, the liquid crystal molecules are arranged in such a fashion that the direction of the spontaneous polarization is in agreement with the direction of the field E. Therefore, the liquid crystal molecules are uniformly arranged at an angle .theta. with respect to the spiral axis 1 (the angle .theta. representing the twist angle of the liquid crystal molecules 2; hereinafter called a "tilt angle") as shown in FIGS. 10(a) and 10(c). An optical shutter element capable of transmitting and cutting off the light can be obtained by utilizing the d.c. field response of the liquid crystal molecules 2.
FIG. 11 shows the structure and principle of operation of a birefringence type optical switch element which transmits and cuts off the light by utilizing the birefringence of the liquid crystal molecules 2. As shown in FIG. 11(a), the birefringence type optical switch element has a structure in which a liquid crystal layer 4 is interposed between, and in parallel with, glass substrates 5a and 5b equipped on the surface thereof a pair of transparent opposed electrodes 6a, 6b, respectively, and two polarization plates 7a and 7b are disposed on both sides of the substrates 5a and 5b in such a fashion that their axes of polarization cross at right angles. In this case, if the axis of polarization of the polarization plate 7a is set to be at an angle .theta. to the spiral axis 1 as shown in FIG. 11(b), the orientation direction of the liquid crystal molecules 2 is in agreement with the axis of polarization of the polarization plate 7a when a negative electric field .vertline.E.vertline.&lt;.vertline.E.sub.c .vertline. is applied, as shown in FIG. 11(d). Therefore, the light 9 incident into the optical switch element from the light source 8 does not pass therethrough but is cut off. When a positive d.c. field (E&gt;E.sub.c) is applied, on the contrary, the orientation direction of the liquid crystal molecules 2 is deviated from the axis of polarization as shown in FIG. 11(c), so that the light passes due to the birefringence effect. In this manner, the optical switching action is attained by reversing the polarity of the d.c. field E, and its response is as fast as from several dozens of .mu.s to several milli-seconds.
If the liquid crystal is used, the thickness of the liquid crystal layer 4 can be reduced to about several .mu.m, and the liquid crystal can be driven at a low voltage of about 10 to about 20 V. In the case of transparent ceramics (PLZT), on the other hand, a voltage as high as about several hundreds of volts is necessary. Hence, the liquid crystal device can be driven at a far lower voltage.
FIG. 12 shows the structure and operation principle of a guest-host type optical switch element for controlling the light transmission quantity by mixing a dichroic pigment into the liquid crystal layer. In the guest-host type element, a dichroic pigment such as a black pigment is placed into the liquid crystal layer. In this case, only one polarization plate is used. The axis of polarization of the polarization plate 7a is arranged as shown in FIG. 12(b). When a negative d.c. voltage (.vertline.E.vertline.&lt;.vertline.E.sub.c .vertline.) is applied as shown in FIG. 12(d), the dichroic pigment assumes the same orientation state as that of the liquid crystal molecules 2, so that the axis of absorption of the dichroic pigment molecules 10 (the major axis of the molecules) is in agreement with the axis of polarization of the polarization plate, and the light 9 incident into the liquid crystal layer 4 is absorbed. Therefore, the light does not pass but is substantially cut off. When a positive voltage (E&gt;E.sub.c) is applied, on the contrary, the orientation direction of the dichroic pigment molecules 10 is deviated from the axis of polarization, so that the light is not absorbed but passes through the optical switch element. In this manner, the guest-host type optical switch element can switch the light by inverting the polarity of the d.c. field E, in the same way as the birefringence type optical switch element (FIG. 11). The element of this type can respond at a high speed by a low voltage in the same way as the birefringence type element.
The optical switch elements using high dielectric materials including the high dielectric liquid crystal are driven by applying thereto a d.c. voltage (field). It has been found, however, that various problems occur in this case due to non-uniform existance of ions. FIG. 13 shows the internal state of the element when a positive d.c. voltage +V.sub.o is applied to the high dielectric liquid crystal element. Since the liquid crystal molecules 2 have the spontaneous polarization P, they are arranged so that the direction of the spontaneous polarization P is in parallel with the field E due to the voltage +V.sub.o applied from outside. The magnitude of the spontaneous polarization is several coulombs (nc)/cm.sup.2 to several dozens of coulombs (nc)/cm.sup.2.
On the other hand, since liquid crystal materials are synthesized, they contain greater amounts of conductive impurities than solid high dielectric materials, and since a high electric field is applied, unstable material compositions are likely to dissociate. Therefore, as shown in FIG. 13(a), those ionic materials which have a negative charge gather close to the orientation film 11a on the side of the positive electrode, and those which have a positive charge gather close to the orientation film 11b on the side of the negative electrode, respectively. Such non-uniform existence of ions increases with a longer application time, and finally gets into saturation (FIG. 13(b)).
FIG. 14 shows the change of a potential distribution state due to the non-uniform existence of ions. The axis of abscissa represents a distance measured from the electrode 6a to which the positive voltage is applied, l.sub.a is a thickness of the orientation film 11a and (l-l.sub.b) is a thickness of the orientation film 11b. The thickness of the orientation films are from several hundreds of angstroms to thousand angstroms (.ANG.), and the thickness of the liquid crystal layer is several microns (.mu.m). When the ionic materials do not exist inside the liquid crystal layer, a potential distribution becomes uniform as represented by a line A, but when non-uniform existence of ions starts occurring as shown in FIG. 13(a), a potential distribution becomes such as B in FIG. 14. As the non-uniformity further proceeds as shown in FIG. 13(b), a potential distribution becomes such as C in FIG. 14. As can be seen from the diagram, non-uniform existence of ions makes the voltage applied to the orientation films higher and makes it more difficult to apply the voltage to the liquid crystal layer. FIG. 15 illustrates the behaviour. As a result, it has been found that the following two problems occur.
First of all, when the voltage applied to the liquid crystal layer drops and when a voltage of the same polarity is continuously applied for a long period, this voltage becomes smaller than the critical voltage V.sub.c, whereby the orientation of the liquid crystal molecules is disturbed and contrast drops (or the performance of the optical switch drops).
Second, the voltage applied to the orientation films increases and an impressed electric field becomes as high as MV/cm. (When non-uniformity of ions does not exist, it is some dozens of KV/cm.) The orientation films are generally organic films, and particularly in the case of films formed by spinner or printing, a large number of pin-holes exist so that the films undergo dielectric breakdown in a high electric field of MV/cm. As a result, the electrode surface comes into direct contact with the liquid crystal at the portions where dielectric breakdown occurs, so that decomposition and degradation of the liquid crystal materials due to the electrochemical reaction proceed. (This means the degradation of the optical switch element.) For the reasons described above, it is necessary to eliminate the non-uniform existence of ions inside the optical switch element which uses a high dielectric liquid crystal (materials).
Though not a method of preventing the non-uniform existence of ions, a driving method is known (European Patent Application: Publication No. 92181) which applies a pulse voltage which prevents the degradation of the high dielectric liquid crystal and determines the desired light transmission state to the liquid crystal element in a predetermined period, and which also applies a voltage signal which makes zero the mean value of the voltages applied in the predetermined period. Though capable of accomplishing the intended objects, this method can not be used as an essential solution method of eliminating the non-uniformity of ions because the voltage which makes the mean value zero is a positive and negative a.c. voltage so that the non-uniformity of ions can exist even when voltage inversion of one cycle is effected.